Integrated front-end passive equalizer and method thereof

ABSTRACT

A passive equalizer circuit incorporated at a front-end of an integrated receiver circuit uses passive components that are distributed between inside and outside of an integrated circuit package. The passive equalizer circuit has off-chip components that are placed on a printed circuit board and on-chip components that are fabricated on a common integrated circuit die as a receiver chip. The on-chip components include one or more variable resistors for adjusting a degree of equalization. The off-chip components include one or more resistors for fine tuning input impedance matching of the integrated receiver circuit.

This application is a continuation of U.S. patent application Ser. No.13/048,292, filed Mar. 15, 2011, titled “Integrated Front-End PassiveEqualizer And Method Thereof”, which is a continuation of Ser. No.12/349,740, filed Jan. 7, 2009, titled “Integrated Front-End PassiveEqualizer And Method Thereof”, which claims the benefit of priorityunder 35 U.S.C. §119(e) of U.S. Provisional Application No. 61/029,259,filed on Feb. 15, 2008 and titled “Integrated Front-End PassiveEqualizer and Method Thereof,” which the entireties of each areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a passive equalizer circuit and inparticular to a passive equalizer circuit that is incorporated in afront-end of an integrated receiver circuit.

2. Description of the Related Art

High-speed serial data transmission is employed in many signalinterfacing standards (e.g., PCI Express, HDMI, and so on). A high-speedserial data transmission system typically includes a transmitter on asource side for generating an electrical signal (e.g., a voltage/currentsignal) to represent serial data based on a two-level signaling scheme(e.g., a high level to represent logically “1” data and a low level torepresent logically “0” data). The high-speed serial data transmissionsystem also includes a transmission line for delivering the electricalsignal from the source side to a destination side and a receiver forreceiving the delivered electrical signal and detecting the serial dataembedded in the electrical signal. The transmission line can be a cable,a PCB (printed circuit board) trace, or a combination of both. Ingeneral, the transmission line behaves similarly to a low-pass filter,which causes more attenuation to a high-frequency component of theelectrical signal than to a low-frequency component of the electricalsignal. As a result, the electrical signal delivered to the destinationside is distorted, and a correct detection of the serial data issometimes difficult unless a proper equalization is performed in thereceiver. Equalization seeks to equalize the overall gain in the entiredata transmission path for different frequency components in theelectrical signal. Since the transmission line behaves similarly to alow-pass filter, a receiver usually employs a high-pass filter forequalization.

In a recent trend, most of the receiver's functions are integrated in asemiconductor integrated circuit (IC). In particular, a receiver ICusually includes an on-chip equalization circuit. Prior art on-chipequalization circuits usually perform equalization using an amplifierwith an inductive load or an RC-degenerated amplifier. Such equalizationschemes usually demand considerable power consumption due to the natureof using active circuits. What is needed is a passive equalizationscheme for an integrated receiver.

SUMMARY OF THE INVENTION

The present invention solves these and other problems by providing amethod and an apparatus for a passive equalizer circuit that ispartially integrated with a receiver chip in a high-speed communicationsystem. One method of equalizing communication signals uses passivecircuits that are distributed between inside and outside of an ICpackage. In one embodiment, the passive equalizer circuit includes afirst passive circuit placed outside of an IC package of the receiverchip (e.g., off-chip) and a shunt circuit fabricated on an IC die withthe receiver chip (e.g., on-chip). The first passive circuit couples afirst input signal from a first transmission line to a first IC packagepin. In one embodiment, the first passive circuit is placed (orassembled) on a PCB and comprises a first resistor coupled in seriesbetween the first transmission line and the first IC package pin. Thefirst IC package pin is coupled to a first IC pad on the IC die insidethe IC package using a first bond wire. The shunt circuit is coupled ina parallel configuration (or in shunt) with a first input of thereceiver chip to the first IC pad. The shunt circuit comprises a seriesresistor-inductor network.

In one embodiment, the high-speed communication system communicatesusing differential signals and the passive equalizer circuit furtherincludes a second passive circuit placed outside the IC package of thereceiver chip. The second passive circuit comprises a second resistorcoupled in series between a second transmission line and a second ICpackage pin. The second IC package pin is coupled to a second IC pad onthe IC die inside the IC package using a second bond wire. The shuntcircuit is coupled across the first and the second IC pads. The secondIC pad is coupled to a second input of the receiver chip, and anequalized differential signal is provided across the first and thesecond inputs (i.e., differential input terminals) of the receiver chipwhen an input differential signal is transmitted across the first andthe second transmission lines (i.e., differential transmission lines).

A transmission line in the high-speed communication system includes acable, a PCB trace, or a combination of both. In some applications, thetransmission line carries signals comprising embedded serial datatransmitted at a rate of 1-10 Giga Bits Per Second. The passiveequalizer circuit is a front-end circuit and can also be configured toprovide impedance matching and AC coupling. For example, a firstcapacitor is coupled in series between the first transmission line andthe first resistor in the first passive circuit (e.g., a first seriesresistor-capacitor circuit) to provide AC coupling. Similarly, a secondcapacitor is coupled in series between the second transmission line andthe second resistor in the second passive circuit (or a second seriesresistor-capacitor circuit) to provide AC coupling. In someapplications, the first capacitor and the second capacitor are eachapproximately 1 nF-1 μF. A combination of the first resistor and one ormore resistors in the shunt circuit can be used to approximately match acharacteristic impedance (e.g., a single-ended impedance) of the firsttransmission line. Similarly, a combination of the first resistor, thesecond resistor, and one or more resistors in the shunt circuit can beused to approximately match a differential impedance across the firstand the second transmission lines.

In one implementation suitable for processing differential signals, theseries resistor-inductor network of the shunt circuit comprises acenter-tapped inductor coupled in series with one or more resistors. Forexample, a third resistor is coupled between the first IC pad and afirst terminal of the inductor while a fourth resistor is coupledbetween the second IC pad and a second terminal of the inductor. Acenter tap of the inductor is coupled to a substantially fixed voltagepotential. In another implementation suitable for processingsingle-ended signals, the series resistor-inductor network of the shuntcircuit comprises an inductor coupled in series with at least oneresistor between the first IC pad and a substantially fixed voltagepotential. The inductor of the series resistor-inductor network has aninductance that is greater than an inductance of a bond wire. In oneembodiment, the inductor's inductance is approximately 3-5 times greaterthan the inductance of the bond wire.

In some applications, the receiver chip has an input impedance that isat least three times higher than an impedance of the shunt circuit. Inone embodiment, one or more of the resistors in the seriesresistor-inductor network are variable (e.g., in response to a controlsignal). For example, the third and the fourth resistor can be varied(or tuned) to adjust a gain difference of the passive equalizer circuitover a frequency range. The third and the fourth resistor also affecttransmission line impedance matching. In one embodiment, the resistorsin the shunt circuit provide coarse tuning for impedance matching whilethe resistors in the first and the second passive circuits provide finetuning for impedance matching. In one implementation, the first and thesecond resistors are each approximately 10 Ohms while the third and thefourth resistors are each approximately 40 Ohms to approximately match100 Ohms differential transmission lines while providing a desired levelof equalization (e.g., a 6 dB gain difference over a 2 GHz frequencyrange).

For purposes of summarizing the invention, certain aspects, advantages,and novel features of the invention have been described herein. It is tobe understood that not necessarily all such advantages may be achievedin accordance with any particular embodiment of the invention. Thus, theinvention may be embodied or carried out in a manner that achieves oroptimizes one advantage or group of advantages as taught herein withoutnecessarily achieving other advantages as may be taught or suggestedherein.

BRIEF DESCRIPTION OF THE DRAWINGS

A general architecture that implements the various features of theinvention will now be described with reference to the drawings. Thedrawings and the associated descriptions are provided to illustrateembodiments of the invention and not to limit the scope of theinvention. Throughout the drawings, reference numbers are re-used toindicate correspondence between referenced elements.

FIG. 1 illustrates one example of a high-speed communication system.

FIG. 2 is a schematic diagram illustrating one embodiment of a passiveequalizer circuit incorporated at a front-end of a receiver forequalizing a differential signal.

FIG. 3 is a graph illustrating transfer characteristics of the passiveequalizer circuit shown in FIG. 2.

FIG. 4 is a graph illustrating return losses of the passive equalizercircuit shown in FIG. 2.

FIG. 5 is a schematic diagram illustrating one embodiment of a passiveequalizer circuit incorporated at a front-end of a receiver forequalizing a single-ended signal.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a passive equalizer and in particularto a passive equalizer that is incorporated in a front-end of anintegrated receiver circuit. While the specification describes severalexample embodiments of the invention considered best modes of practicingthe invention, it should be understood that the invention can beimplemented in many ways and is not limited to the particular examplesdescribed below or to the particular manner in which any features ofsuch examples are implemented. In some instances, well-known details arenot shown or described to avoid obscuring aspects of the invention.

FIG. 1 illustrates one example of a high-speed communication system witha passive equalizer 100 incorporated at a front-end of a receiver 102with an IC receiver chip 101. A driver circuit 104 outputs adifferential signal (VOUT+, VOUT−) at a source (e.g., a transmitter 105)that is communicated to a destination (e.g., the receiver 102) usingtransmission lines 103. The transmission lines 103 may include cables,PCB traces or other transmission mediums that attenuate the differentialsignal from the source in a non-uniform manner across a desiredfrequency range (e.g., with higher losses for higher frequencycomponents of the differential signal). The passive equalizer 100 at thedestination receives the attenuated differential signal (VIN+, VIN−)from the transmission lines 103 and compensates for the non-uniformattenuation (or frequency distortion) to generate an equalizeddifferential signal (VE+, VE−) for the IC receiver chip 101.

In one configuration, the passive equalizer 100 uses a distributednetwork of components with some passive components integrated on acommon IC die with the IC receiver chip 101 and other passive components(e.g., discrete parts) placed outside of the IC receiver chip 101 on aprinted circuit board (PCB) for the receiver 102. The passive componentsoutside the IC receiver chip 101 are connected to the passive componentson the common IC die by IC package pins, bond wires, and IC pads. The ICpads are effectively capacitive (e.g., 0.2 pF-1 pF) and greatly limitshigh frequency performance. The bond wires are effectively inductive(e.g., 1 nH-10 nH). In this configuration, the passive equalizer 100accounts for the capacitive effects of IC pads and the inductive effectsof bond wires to improve high frequency performance. The passiveequalizer 100 can also provide AC coupling and impedance matching forthe transmission lines 103.

FIG. 2 is a schematic diagram illustrating one embodiment of a passiveequalizer incorporated at a front-end of a receiver for equalizing aninput differential signal received from a pair of transmission lines103. The passive equalizer (or front-end circuit) comprises a part in aPCB 110, a part in an IC package 120, and a part in an IC die 130 with areceiver chip 101. For example, the part in the PCB 110 comprises twoseries resistor-capacitor (RC) circuits 111, 112, the part in the ICpackage 120 comprises IC package pins 124, 121 and bond wires 122, 123,and the part in the IC die 130 comprises IC pads 131, 132 and a shuntcircuit 133.

In one embodiment, the pair of series RC circuits 111, 112 placedoutside of the IC package is configured to electrically couple the pairof transmission lines 103 to the pair of IC package pins 124, 121. Forexample, the first series RC circuit 111 comprises a first capacitor C1a and a first resistor R1 a connected in series between the firsttransmission line 103 a and the first IC package pin 124. The secondseries RC circuit 112 comprises a second capacitor C1 b and a secondresistor R1 b connected in series between the second transmission line103 b and the second IC package pin 121. The first transmission line 103a and the second transmission line 130 b are configured as a pair toprovide a differential voltage signal comprising a positive end (VIN+)and a negative end (VIN−).

The first capacitor C1 a and the second capacitor C1 b function to blocka DC component of the differential voltage signal. It is acceptable toremove the DC component since it carries no useful information. It isalso desirable to remove the DC component since the differential voltagesignal's DC level may be incompatible with a voltage range employed bythe receiver chip 101. The first resistor R1 a and the second resistorR1 b work with the shunt circuit 133 to provide a desired level ofequalization and impedance matching. In particular, the first resistorR1 a and the second resistor R1 b facilitate fine adjustments for aninput impedance of the receiver to approximately match a characteristicimpedance (Z₀) of the transmission lines 103. In one embodiment, thefirst resistor R1 a and the second resistor R1 b are variable (e.g., atmanufacturing or during operation in response to a control signal). Theseries RC circuits 111, 112 can be fabricated on the PCB or can bediscrete components assembled on the PCB.

The pair of bond wires 122, 123 in the IC package 120 are electricallycoupled between the pair of IC package pins 124, 121 and the pair of ICpads 131, 132 on the IC die 130 inside the IC package 120. In oneembodiment, the shunt circuit 133 in the IC die 130 comprises a seriesresistor-inductor network that is electrically coupled between the pairof IC pads 131, 132. An equalized differential signal (VE+, VE−) isprovided across the pair of IC pads 131, 132 and provided to adifferential input of the receiver chip 101. The seriesresistor-inductor network 133 and receiver processing circuits of thereceiver chip 101 are fabricated on the same IC die 130.

In one embodiment, the series resistor-inductor (R-L) network 133comprises a pair of tunable resistors R2 a, R2 b and a center-rappedinductor L. For example, the first tunable resistor R2 a is coupledbetween the first IC pad 131 and a first terminal of the inductor L. Thesecond tunable resistor R2 b is coupled between the second IC pad 132and a second terminal of the inductor L. The center-tap of the inductorL is coupled to a substantially fixed-potential node on the IC die 130(e.g., VDD or other substantially fixed supply voltage).

The equalized differential signal (VE+, VE−) across the pair of IC pads131, 132 sees a shunt impedance comprising the tunable resistors (R2 a,R2 b) and the center-tapped inductor L of the series R-L network 133. Asdescribed above, the equalized differential signal is provide to thedifferential input of the receiver chip (or on-chip receiver) 101. Thedifferential input of the receiver chip 101 has an input impedance thatis substantially higher (e.g., at least 3 times higher) than the shuntimpedance. Thus, the series R-L network 133 exhibits a high-passresponse and effectively realizes an equalization function with a degreeof equalization mostly determined by a time constant of the series R-Lnetwork 133. In one embodiment, the first tunable resistor R2 a and thesecond tunable resistor R2 b have approximately equal resistance values(e.g., R_(e)). If the inductor L has an inductance of L_(e), then thetime constant (T) of the series R-L network 133 is approximatelyT=L_(e)/(2R_(e)).

A greater time constant T leads to a higher relative boost to a highfrequency component in the equalized differential signal. Thus, thedegree (or level) of equalization can be adjusted by adjusting the timeconstant T. For example, the time constant T may be increased toincrease the degree of equalization in response to increased highfrequency attenuation in the transmission lines 103 and/or IC packaging.The time constant T may be adjusted by adjusting the inductor L and/orthe tunable resistors R2 a, R2 b. The inductance of the inductor L isequal to or greater (e.g., 3-5 times greater) than the inductance of thebond wires 122, 123. In one embodiment, the inductance of the inductor Lis not adjusted while the tunable resistors R2 a, R2 b are varied tochange the degree of equalization for the passive equalizer. The tunableresistors R2 a, R2 b can be varied in response to a control signal or bynumerous other methods known to those of ordinary skill in the art andare thus not described in further detail.

As discussed above, the passive equalizer also functions toapproximately match the input impedance of the receiver to thecharacteristic impedance of the transmission lines 103. In oneembodiment, the resistance value of the first resistor R1 a is adjustedsuch that a combined resistance of the first resistor R1 a and the firsttunable resistor R2 a is approximately equal to a single-endedcharacteristic impedance of the first transmission line 103 a.Similarly, the resistance value of the second resistor R1 b is adjustedsuch that a combined resistance of the second resistor R1 b and thesecond tunable resistor R2 b is approximately equal to a single-endedcharacteristic impedance of the second transmission line 103 b. Thus,the passive equalizer uses a combination of external resistors (e.g.,the first resistor R1 a and the second resistor R1 b) and on-chipresistors (e.g., the first tunable resistor R2 a and the second tunableresistor R2 b) to provide impedance matching at different levels ofequalization.

By way of example, FIG. 3 illustrates waveforms from a circuitsimulation showing two different levels of equalization for twodifferent sets of resistor values in the passive equalizer while FIG. 4shows associated return losses that indicate similar impedance matchingfor both sets of resistor values. In this circuit simulation, thesingle-ended characteristic impedance (Z₀) of the transmission lines 103is about 50 Ohms, the bond wire inductance is about 2.5 nH, the padcapacitance is about 0.25 pF, and the inductor L is about 20 nH.

A graph 300 in FIG. 3 shows the transfer characteristic of the passiveequalizer with respect to frequency for the first set of resistor values(e.g., R1 a=R1 b=0 Ohm, R1 a=R2 b=50 Ohms). A graph 302 shows thetransfer characteristic of the passive equalizer with respect tofrequency for the second set of resistor values (e.g., R1 a=R1 b=10 Ohm,R1 a=R2 b=40 Ohms). The time constant for the second set of resistorvalues is higher so the level of equalization is greater for the secondset of resistor values. For example, the first set of resistor valuesprovides about a 4 dB boost while the second set of resistor valuesprovides about a 6 dB boost with respect to DC at a high frequency pointof interest (i.e., 2 GHz).

A graph 400 in FIG. 4 shows the return loss of the passive equalizerwith respect to frequency for the first set of resistor values and agraph 402 shows the return loss for the second set of resistor values.For most of the frequency range, including 2 GHz, the return loss isapproximately the same. Thus, the passive equalizer has similarimpedance matching for both the first and the second sets of resistorvalues. In other words, the external resistors (e.g., R1 a and R1 b)help to preserve the passive equalizer's performance in impedancematching for different levels of equalization.

The principle of the present invention can be practiced in manyalternative embodiments without departing from the scope of the presentinvention. For example, the passive equalizer is not limited to adifferential signaling scheme. FIG. 5 is a schematic diagramillustrating one embodiment of a passive equalizer incorporated at afront-end of a receiver for equalizing a single-ended signal (VIN).Similar to the passive equalizer of FIG. 2, the passive equalizer inFIG. 5 has parts distributed among a PCB 110, an IC package 120, and anIC die 130. For example, an external passive circuit 111 comprising a DCblocking capacitor C1 coupled in series with a resistor R1 is placed onthe PCB 110 to connect a transmission line 103 to an IC package pin 124in the IC package 120. A bond wire 122 connects the IC package pin 124to an IC pad 131 on the IC die 130. A shunt circuit 143 is connected ina parallel configuration with an input of a receiver chip 101. The shuntcircuit 143 and the receiver chip 101 are fabricated on the common ICdie 130.

In one embodiment, the shunt circuit 143 comprises an inductor L coupledin series with a variable resistor R2 between the IC pad 131 and asubstantially fixed voltage potential (e.g., VDD or AC ground). Acombination of the variable resistor R2 and the resistor R1 is used toprovide impedance matching. The inductor L and the variable resistor R2establishes a time constant for the shunt circuit 143. The variableresistor R2 is adjustable to change a gain difference of the passiveequalizer over a frequency range (i.e., degree of equalization). Theresistor R1 can be adjusted to preserve impedance matching for differentdegrees of equalization.

While certain embodiments of the inventions have been described, theseembodiments have been presented by way of example only, and are notintended to limit the scope of the inventions. Indeed, the novel methodsand devices described herein may be embodied in a variety of otherforms; furthermore, various omissions, substitutions, and changes in theform of the methods and devices described herein may be made withoutdeparting from the spirit of the inventions. The accompanying claims andtheir equivalents are intended to cover such forms or modifications aswould fall within the scope and spirit of the inventions.

1. A distributed passive equalizer for a receiver chip comprising: afirst part in a printed wiring board for a receiver chip; a second partin an IC package for the receiver chip; and a third part in an IC diewith the receiver chip, the first part coupled in series between a firsttransmission line and the second part, the second part including a firstIC package pin, a first IC pad, and a first bond wire, the third part ina parallel configuration with a first input of the receiver chip to thesecond part.
 2. The distributed passive equalizer of claim 1 wherein thefirst part includes a first passive component.
 3. The distributedpassive equalizer of claim 2 wherein the first passive component couplesin series between the first transmission line and the first IC packagepin.
 4. The distributed passive equalizer of claim 1 wherein the firstbond wire couples the first IC package pin and the first IC pad on theIC die inside the IC package.
 5. The distributed passive equalizer ofclaim 1 wherein the third part includes a shunt circuit coupled in aparallel configuration with the first input of the receiver chip to thefirst IC pad.
 6. The distributed passive equalizer of claim 5 whereinthe shunt circuit includes a series passive component network betweenthe first IC pad and a substantially fixed voltage potential.
 7. Thedistributed passive equalizer of claim 1 wherein the first part, thesecond part, and the third part are used to provide impedance matchingof the first transmission line and to generate an equalized signal atthe first input of the receiver chip.
 8. The distributed passiveequalizer of claim 7 wherein the first part includes a first seriespassive circuit coupled in series between the first transmission lineand the first IC package pin, and the third part includes a seriespassive component network formed on the IC die with the receiver chip ina parallel configuration with the first input of the receiver chip tothe first IC pad.
 9. The distributed passive equalizer of claim 1wherein the third part and receiver processing circuits of the receiverchip are fabricated on the IC die.
 10. The distributed passive equalizerof claim 1 further comprising: a fourth part in the printed wiring boardfor the receiver chip; a fifth part in the IC package for the receiverchip; and a sixth part in the IC die with the receiver chip, the fourthpart coupled in series between a second transmission line and the fifthpart, the fifth part including a second IC package pin, a second IC pad,and a second bond wire, the sixth part in a parallel configuration witha second input of the receiver chip to the fifth part such that anequalized differential signal is provided across the first and secondinputs of the receiver chip when an input differential signal istransmitted across the first and second transmission lines.
 11. Thedistributed passive equalizer of claim 10 wherein the fourth partincludes a second passive component coupled in series between the secondtransmission line and the second IC package pin.
 12. The distributedpassive equalizer of claim 10 wherein the second bond wire couples thesecond IC package pin and the second IC pad on the IC die inside the ICpackage.
 13. The distributed passive equalizer of claim 10 wherein thesixth part includes a shunt circuit coupled in a parallel configurationwith the second input of the receiver chip to the second IC pad.
 14. Thedistributed passive equalizer of claim 13 wherein the shunt circuitincludes a series passive component network between the second IC padand a substantially fixed voltage potential.
 15. A method of equalizingtransmitted signals using distributed passive circuits, the methodcomprising: coupling in series a first passive part in a printed wiringboard for a receiver chip between a first transmission line and a secondpassive part in an IC package for the receiver chip, the second passivepart including a first IC package pin, a first IC pad, and a first bondwire coupling the first IC package pin to the first IC pad; and couplinga third passive part in an IC die with the receiver chip in a parallelconfiguration with the second passive part to a first input of thereceiver chip.
 16. The method of claim 15 wherein the first passive partincludes a first passive component coupled in series between the firsttransmission line and the first IC package pin.
 17. The method of claim15 wherein the third passive part includes a shunt circuit coupled in aparallel configuration with the first input of the receiver chip to thefirst IC pad on the IC die inside the IC package.
 18. The method ofclaim 17 wherein the shunt circuit includes a series passive componentnetwork.
 19. The method of claim 15 further comprising providingimpedance matching of the first transmission line using the first andthird passive parts.
 20. The method of claim 19 further comprisinggenerating an equalized signal at the first input of the receiver chipusing the first and third passive parts.